1. Field of the Invention
The present invention relates to a monolithic electronic element fabricated from a semiconducting ceramic and, more particularly, to a monolithic electronic element fabricated from a semiconducting ceramic (hereinafter may be referred to as monolithic electronic element) having a positive temperature coefficient of resistance.
2. Background Art
Semiconducting electronic elements having a positive temperature coefficient of resistance (hereinafter referred to as a PTC characteristic), meaning that electrical resistance increases drastically when temperature exceeds the Curie temperature, have been used to protect a circuit from overcurrent or to control degaussing module of a color television set. In view of their advantageous PTC characteristic, semiconducting ceramics predominantly comprising barium titanate have generally been used in such semiconducting electronic elements.
However, in order to make barium titanate-based ceramics semiconducting, firing must generally be performed at a temperature of 1300xc2x0 C. or more. Such treatment at high temperature has the following drawbacks: a tendency to damage a furnace used for firing; a high cost of maintaining the furnace; and high energy consumption. Thus, there has been demand for semiconducting ceramics comprising barium titanate which can be fired at lower temperature.
To overcome the above drawbacks, a modified technique is disclosed in xe2x80x9cSemiconducting Barium Titanate Ceramics Prepared by Boron-Conducting Liquid-Phase Sinteringxe2x80x9d (In-Chyuan Ho, Communications of the American Ceramic Society, Vol. 77, No. 3, P829-p832, 1994). Briefly, the temperature at which the ceramics exhibit semiconduction is lowered by addition of boron nitride to barium titanate. The literature reports that the boron nitride-added ceramics can become semiconducting at a firing temperature of about 1100xc2x0 C.
Meanwhile, in recent years there has been a demand for monolithic chip-type semiconducting ceramic electronic elements which attain low resistance and high withstand voltage at ambient temperature and are suitable for high-density packaging.
Generally, a monolithic chip-type semiconducting ceramic electronic element is produced by stacking ceramic green sheets and internal electrode paste layers alternatingly and firing the same together in a firing furnace. Therefore, a base metal such as nickel is used for producing internal electrodes, in that such a base metal can establish ohmic contact with a ceramic material even when the metal is simultaneously fired with the ceramic material. When fired in air, such a base metal is oxidized. Thus, the stacked body is fired in a reducing atmosphere and then reoxidized at a temperature at which internal electrodes are not oxidized, to thereby fire a semiconductor ceramic material and an internal electrode material together. However, reoxidation carried out at a relatively low temperature is detrimental to a PTC characteristic of the thus-fired product.
Japanese Patent Application Laid-Open (kokai) No. 8-153605 discloses a method for attaining a PTC characteristic even when reoxidation is carried out at low temperature. The method employs a perovskite compound in a form of microparticles as a titanium salt serving as a predominant component. Use of the perovskite compound enables sintering at a temperature as low as 1000xc2x0 C.-1250xc2x0 C. and provision of a PTC characteristic even when reoxidation is carried out at a temperature as low as 500xc2x0 C. or higher.
However, a conventional monolithic electronic element must be produced through reoxidation at approximately 1000xc2x0 C. so as to obtain a satisfactory PTC characteristics, and internal electrodes might be oxidized. Therefore, there has been demand for a monolithic electronic element which can be produced through firing at low temperature so as to obtain a satisfactory PTC characteristic through reoxidation at a temperature lower than that employed conventionally.
In view of the foregoing, the present invention is directed to provision of a monolithic electronic element which can be produced through firing at 1000xc2x0 C. or lower and exhibits a satisfactory PTC characteristic even when the element is produced through reoxidation at low temperature.
Accordingly, the present invention provides a monolithic electronic element fabricated from a semiconducting ceramic, which element comprises a sintered laminate formed of alternatingly stacked semiconducting ceramic layers and internal electrode layers, and external electrodes formed on the sintered laminate, wherein each semiconducting ceramic layer comprises semiconducting sintered barium titanate containing the following substances: boron oxide and an oxide of at least one metal selected from among barium, strontium, calcium, lead, yttrium and a rare earth element; a portion of the titanium being optionally replace by at least one metal selected from among tin, zirconium, niobium, tungsten and antimony, the boron oxide being incorporated in an amount, as reduced to atomic boron, satisfying the following relationships:
0.001xe2x89xa6B/xcex2xe2x89xa60.50
and
0.5xe2x89xa6B/(xcex1xe2x88x92xcex2)xe2x89xa610.0
wherein xcex1 represents the total number of atoms of barium, strontium, calcium, lead, yttrium and a rare earth element contained in the semiconducting ceramic, and xcex2 represents the total number of atoms of titanium, tin, zirconium, niobium, tungsten and antimony contained in the semiconducting ceramic.
The semiconducting ceramic having such a composition can be fired at 1000xc2x0 C. or lower and exhibits its improved PTC characteristic even when the ceramic is subjected to reoxidation at low temperature. Thus, a base metal can be employed as an internal electrode, and a satisfactory PTC characteristic is realized.
Preferably, the monolithic electronic element fabricated from a semiconducting ceramic contains a donor element and an acceptor element, these elements being incorporated in amounts satisfying the following relationships:
xe2x80x830.0001xe2x89xa6Md/xcex2xe2x89xa60.005
and
0.00001xe2x89xa6Ma/xcex2xe2x89xa60.005
wherein Md represents the total number of atoms of a donor element in the semiconducting ceramic layers, Ma represents the total number of atoms of an acceptor element in the semiconducting ceramic layers, and xcex2 represents the total number of atoms of titanium, tin, zirconium, niobium, tungsten and antimony contained in the semiconducting ceramic.
The ceramic having such a composition provides a monolithic electronic element which exhibits its PTC characteristic very effectively.